A choke valve is a throttling device used to reduce the fluid pressure or flow rate in a flow line. (Hereinafter the device is referred to as a "choke".) Chokes are commonly used in connection with well drilling and production operations for producing oil or gas, although they have other applications as well.
In many cases, the stream passing through the choke is very erosive. For example, a choke used in a drilling or production operation may be exposed to a high pressure gas stream moving at high velocity and carrying sand particles. In the course of undergoing pressure reduction in passing through the choke, the velocity of the flow stream is increased. This exacerbates the erosional effect of the flow stream. Choke failure due to erosion can lead to very serious problems in controlling well flow or, as a minimum, leads to costly repairs.
As a result of these factors, designers of chokes go to great lengths in trying to improve the durability of their products. The present invention is concerned with a combination of design features which, when implemented, yielded surprisingly beneficial durability improvements in a type of known choke commonly referred to as having a "cage with external sleeve flow trim".
In general, such a choke comprises:
a valve body and a flow trim; PA1 the valve body having an axial main bore or cavity extending part way through the body, an inlet bore connecting with the inner end of the main bore from the side, and an outlet bore extending to one end of the body from the main bore. The three bores form a generally T-shaped bore network wherein the main bore contains the valve components; PA1 the inlet and outlet passages into the main bore may take various configurations to provide appropriate flow line interface; and PA1 the flow trim, comprising a cage and external throttling sleeve, being positioned in the main bore at the junction of the bores. The cage is a stationary tubular member which extends transversely over the inner end of the inlet bore. The cage typically has two pairs of ports, arranged in opposed relationship, extending through its side wall. The ports of one pair are larger than those of the other pair. The flow stream may move through the inlet bore, enter the cage through the ports and leave through the outlet bore. An annular passageway is formed between the body and the cage so that fluid entering from the inlet bore may circulate through the passageway to reach the various ports. The tubular throttling sleeve is configured to closely fit over the cage and it can be advanced axially to slide across the ports and throttle or close them as required. PA1 (1) One pair of ports is aligned with the axis of the inlet bore; and PA1 (2) A protuberance, referred to as a deflection bar, extends downwardly and inwardly from the body into the main bore in alignment with its axis and located about 180.degree. from the axis of the inlet bore. Otherwise stated, the bar is located at the top of the main bore, in line with the axis of the inlet bore. The deflection bar preferably having angled side surfaces for deflecting the circularly flowing streams moving through the semi-circular branches of the annular passageway, whereby they converge together angularly instead of head on.
There are, of course, additional necessary components, such as means for advancing or withdrawing the throttling sleeve and means for closing and sealing the main bore, but they are not involved in the present invention and will not be described in detail herein.
In summary therefore, the choke design of interest utilizes a stationary cage coupled with a sliding external sleeve to control the flow area (the ports). The choke is designed so that the stream entering through the inlet bore travels into the cage through the annular passageway and the throttled ports; it then leaves through the outlet bore.
There are a number of chokes available commercially which utilize the aforementioned flow trim design. U.S. Pat. No. 4,540,022 ("'022") discloses one such choke having a flow trim comprising a cage and external sleeve. This patent is assigned to the assignee of the present application. The present invention was developed in the course of a research effort to extend or improve the durability of this choke.
It needs to be noted that the '022 choke, in its commercially offered form, has always had the ports offset 45.degree. from the axis of the inlet bore.
It further needs to be understood that, in some models of the choke of interest, an internal sleeve (referred to as a retainer) is provided, contiguous to the main bore surface. This retainer acts to lock the cage in place. For purposes of the claims of this application, the retainer is to be considered to be simply an extension of the body. So when the term "body" is used in the claims, it is intended to describe both a body without an internal retainer or one equipped with a retainer.